Incorporated patch from haixia@ changing the default implementation of
array index clamping to use the clamp intrinsic. This works more
reliably on pure OpenGL ES devices and on Windows.

Added a mechanism in ShBuiltInResources to choose the strategy for
array index clamping.

BUG=none
TEST=various out-of-bounds array indexing tests and various WebGL content
Review URL: https://codereview.appspot.com/7194051

git-svn-id: http://angleproject.googlecode.com/svn/trunk@1733 736b8ea6-26fd-11df-bfd4-992fa37f6226
Changed optimization level to D3DCOMPILE_OPTIMIZATION_LEVEL1. With
this optimization level, it appears that array index clamping can be
enabled on Windows without destabilizing WebGL content.

Tested this change with array index clamping enabled with MapsGL, a
previous failing test from AlteredQualia, many Three.js demos, the
Julia Set tutorial from acko.net, the WebGL 1.0.1 conformance suite,
and other content; all worked.

BUG=none
Review URL: https://codereview.appspot.com/7197054

git-svn-id: http://angleproject.googlecode.com/svn/trunk@1734 736b8ea6-26fd-11df-bfd4-992fa37f6226